Understanding and maintaining an effective lubrication system

For gearing equipment owners and operators, the ultimate
goal is to achieve a return on their investment; this is done
by both maximizing the output, reliability and efficiency of their
machinery, and minimizing downtime and operating costs.

Continued reliability, successful operation
and long life of power transmission equipment largely depend
upon the constant supply of lubrication oil of proper quantity,
quality and condition. The lifeline of the gearbox is its
lubrication system, critical for supporting the drive under all
modes of operation.

The purpose of a gearbox lubrication system is to provide an
oil film at the contacting surfaces of all working components
to reduce friction and wear. In addition, the oil serves to
remove and dissipate heat from where it is generated,
preventing gearing component temperatures from rising to
excessive levels. Other lubrication functions include the
transfer and/or removal of wear particles, as well as the
filtration of rust and corrosion and any other undesirable
contaminants.

However, failure of the lubrication system to perform any
one or more of these functions may result in premature
equipment failure.

Understanding the role and importance of a lubrication
system in the overall life of a gearbox serves as a foundation
for understanding the needs for maintaining such an effective
system. And that is what this article aims to doprovide
maintenance professionals with the
tools to properly understand the lubrication needs for
extending overall life of their gearbox. This article examines
the number of lubricant types available, as well as the systems
used to supply such lubricant throughout a gearbox. In
addition, proper maintenance functions are provided for
sustaining a functional, effective lubrication system.

Understanding lubrication. Lubrication can
be defined as the control of friction and wear between adjacent
surfaces by the development of a lubricant film between them,
called an elastohydrodynamic (EHD) oil film.

EHD film thickness between gear tooth surfaces is quite
small, usually less than 1.25 micrometers (0.00005 in.). Oil
film thickness is significantif the adjacent surfaces are
not fully separated, the EHD film leaves local areas of contact
between those surfaces, making them vulnerable to surface
fatigue.

Viscosity is a characteristic of fluids to resist flowing
freely. It is one of the most important characteristics of a
lubrication fluid. Lubricating oil viscosity changes
appreciably with temperature, and is generally stated at two
temperatures: 40°C (100°F) and 100°C (210°F).
Viscosity is usually expressed in terms of the time required
for a standard quantity of a fluid at a given temperature to
flow through a standard opening.

Fatigue life of contacting components of a gearbox, such as
gear teeth and bearing rollers, is determined by a complex
combination of speed, load, lubricant temperature, clearance
and alignment. The lubricant's role in this interaction is
determined primarily by speed, viscosity and temperature. The
effect of these factors on the fatigue life of elements can be
dramatically altered at higher temperatures with lower
viscosity, and thinner resultant oil films. Selecting the
correct lubricant for any application requires a careful study
of expected operational and environmental conditions.

Gear lubricants. Several factors must be
considered before choosing a gear lubricantthe unit's
operating speed and load, temperature range and lubricant
availability, to name a few. However, the most important
parameter in selecting a lubricant is viscosity. High-speed
units produce an acceptable oil film at the tooth contact area
even with a low-viscosity oil; at lower operating speeds, a
thinner oil film is generated, requiring more viscous oils to
separate contacting surfaces. Still, often a gearbox will
contain both high- and low-speed gear meshes. In these cases, a
compromise must be obtained (though in such cases, performance
of these gear meshes may be reduced).

There are two basic types of lubricants used in gear drive
systems: petroleum-based mineral oils and a general category
known as synthetic lubricants.

Petroleum-based lubricants. Petroleum-based
mineral oils are complex mixtures derived from refining crude oil. Petroleum
products have been found to excel as lubricants in most
applications. Mineral oils are usually compounded with
different chemical additives to improve specific properties
such as increased lubricant life, resistance to rust and
oxidation and even increased load-carrying capacity.

High-load oils, called extreme-pressure (EP) gear
lubricants, contain selected additives that increase the
load-carrying capacity of gearing by forming a film on the
metal that provides component separation under higher load
conditions. EP lubricants are ideal for use when severe
operating conditions are anticipated. Often these lubricants
will contain more than one chemical additive for load capacity
enhancement over a wide temperature range, most commonly
compounds of phosphorous and sulfur. However, EP gear
lubricants should not be used in gear units containing an
internal backstop or an internal friction clutch unless the
lubrication types used have been specifically approved by the
gearbox manufacturer.

Care must be taken when synthetic lubricants are substituted
for previously utilized lubricants. Compatibility with other
gearbox components like rubber lip seals, rubber O-ring seals
and housing paint must be established. Synthetic lubricants can
be up to four times more costly than petroleum-based oils, and
are thus generally reserved for problem applications such as
extremely high or low temperatures, equipment subjected to
frequent overloads and equipment with a marginal lubrication
system.

The largest class of today's synthetic lubricants is the
estersmaterials containing the ester chemical linkage.
Esters have wide operating temperature ranges and high
viscosity indicesthus permitting low-temperature
operation, as well as providing good lubrication
characteristics at high temperatures. A lubricant's viscosity
index is a measure of how much that oil's viscosity varies with
temperature.

Another class of synthetic lubricants is the synthesized
hydrocarbonsthese lubricants contain many of the
advantages of esters (to a lesser extent), but have a similar
structure to mineral oils, making them compatible with mineral
oils while not being detrimental to seals and paints (esters
have low compatibility with some polymeric materials such as
those used in seals and paints).

Lubricant viscosity selection. In general,
the lowest viscosity oil capable of forming an adequate oil
film at all operating conditions should be chosen. However, in
practice, the lubricant chosen is often a compromise between
the requirements of the various lubricated componentssuch
as gears and bearingsand the particular application
requirements such as large ambient temperature
differentials.

Lubrication systems. There are two types of
gearbox lubrication systems in use: splash lubrication systems
and force-feed lubrication systems. The intent of both types of
systems are the same, to distribute oil to each component of
the gearbox sufficient for lubricating and cooling that
component yet minimizing heat generation by oil churning.

Splash lubrication. Splash lubrication
systems require that the gearbox be filled to a predetermined
lubrication oil level. Rotating gear elements within the
gearbox must dip into the oil and "sling" it into troughs,
pockets or directly to bearings and gear meshes requiring
lubrication and cooling oil. Feed troughs are employed to
capture oil that is "slung" onto the upper gearbox housing wall
by a dipping gear element. This oil drips into the trough
which, in turn, distributes that oil to the bearings. Such
systems are better suited for gearboxes containing
rolling-element bearings than those with journal bearings,
which require far more oil.

A splash lubrication system requires at a minimum, oil
troughs, bearing oil pockets, an oil fill location, an oil
drain and a breather. In cold ambient temperatures, an
immersion heater should be provided in the sump. Cold starting
temperatures can cause oil viscosity to be too high to properly
distribute oil upon startup.

Splash lubrication systems are far simpler and less
expensive than force feed, but are applicable only to low-speed
gear units. As shaft operational speeds increase, the heat
generated in the gearbox becomes excessive, requiring an
external, force-feed system to supply larger volumes of
lubricant to lubricate and cool gearbox components. In
addition, higher-speed units require oil to be precisely
introduced at the gear and bearing interfaces; this is
accomplished through strategically placed jets to properly
lubricate the gear meshes and dedicated bearing oil supply
lines.

Temperature control/thermal rating. The
second primary function of the lubrication system is to provide
heat removal. Adequate cooling is necessary to maintain oil
viscosity control and oil quality. Conversely, for every gear
drive there is a thermal rating; the average power that can be
transmitted continuously without overheating the unit and
without using any special external cooling method. If the
thermal rating is less than the mechanical ratingthe load
a gearbox can transmitadditional cooling supplied by a
force-feed lubrication system is required.

Auxiliary cooling can be used in combination with splash
lubrication to increase the thermal rating of a
gearboxfor instance, air can be forced past the radiating
surfaces of the gear casing by strategically placed fans
internal to the gearbox and located on a high-speed pinion
shaft. In addition, the unit can be cooled by a water jacket;
water passages are built into the gear housing, usually at the
high-speed end, and heat is carried away by a cooling water
flow that is isolated from the lubrication oil sump.

To operate a gear unit at maximum efficiency, auxiliary
cooling schemes should include thermostatic controls so that
the oil is not cooled unnecessarily. Operating with too cool a
lubricant increases churning losses. Adding cooling fins to
increase the surface area of the gearbox casing can increase
the heat transfer from the gear casing to the ambient air.

Force-feed lubrication. In a typical
force-feed lubrication system, a shaft- or a motor-driven oil
pump draws oil from the gearbox sump through a suction pipe.
The oil is directed from the pressure side of the oil pump
through a filter to cleanse the oil, and through a cooler
employed to cool the oil. A pressure relief valve is typically
located before this filter to protect the system from too high
an operating pressure. If the filter becomes clogged, the
relief valve will permit the unfiltered oil to bypass the
filter so the gearbox will continue to receive lubrication and
cooling oil albeit unfiltered (unfiltered oil is better than no
oil). Another relief valve is often located at the inlet to the
gearbox to limit the oil feed pressure if the system contains
both shaft- and motor-driven pumps, and both are running at the
same time. A shaft-driven oil pump is driven by one of the
rotating gear shafts of the gearbox. Some lubrication systems
will include both motor- and shaft-driven oil pumps. The
motor-driven pump can be activated prior to gearbox startup to
supply full oil flow requirement to the gearbox prior to shaft
rotation until the attached lube oil pump is running at a speed
sufficient to supply full lubrication oil flow to the gearbox,
during gearbox coast-down or as a backup in case of failure of
the main shaft-driven oil pump.

Check valves are located so that the main pump does not pump
through the auxiliary system and that the auxiliary pump does
not pump into the pressure side of the main oil pump. A bypass
is provided at the cooler serving as both a pressure relief
valve and/or a thermostatically controlled valve set so that
the pressure drop across the cooler is limited during times
when the oil is cold; additionally, temperature and pressure
sensors are located at various critical points throughout the
system.

Relatively little oil is required for lubrication using a
force-feed system provided it is properly applied. The bulk of
the oil flow is required for cooling the gear tooth flanks and
bearings. For demanding high-speed applications, gear tooth
meshes are sprayed on either in-mesh or out-mesh sides or, in
some instances, both sides.

System components. A typical force-feed
lubrication system consists of the following major
components:

Pumps. The gearbox oil pump delivers a
given quantity of oil over a wide range of oil temperatures and
viscosities. In addition, the gearbox pump must be capable of
priming itself and overcoming pressure drops in the line
between the oil reservoir and the pump suction port.

The most common method of lubricant delivery is the
positive-displacement lube-oil pumpthese pumps deliver a
given quantity of fluid with each pump rotor revolution. A
positive-displacement pump's output is directly proportional to
its operating speed and offers practically constant oil flow at
any particular speed regardless of downstream conditions.

Gearbox lubrication pumps can be mounted to the unit and
driven by one of the gearbox shafts, or independently mounted
with an electric motor or other prime mover driving. When the
pump is shaft driven, oil flow will vary directly with shaft
speed. In a gear pump, as the gears rotate, fluid is trapped
between the gear teeth and the case, and is carried around the
pump casing to the pump discharge oil port.

Filtration. Gearbox lubrication systems are
subject to contamination due to a variety of
causesinternal component wear generates particles washed
into the oil stream; foreign particles find their way into the
system during assembly, maintenance and everyday operation.
These contaminants, if uncontrolled, will cause wear and even
failure of bearings or gear elements.

Lubricant cleanliness is a major concern when looking to
maximize geared equipment service life. The lubrication filters
play a key role in ensuring that abrasive particles are removed
from the system.

In addition to the filtration of fluids, the lubrication
filters often incorporate a bypass for clogged element
conditions, a magnetic drain plug to collect metallic particles
and a visual and/or electrical cleanliness indicator.

In force-feed lubrication systems, the oil must be supplied
through a filter media that is compatible with the lubricant,
meets the viscosity requirements without excessive pressure
drop and removes particulate matter consistent with the
rotating equipment design.

The oil filter should be located on the pressure side of the
pump so warmer, less viscous oil is being filtered. Filter
elements can be either cleanable and reusable or disposable.
Cleanable filter elements are usually made of wire mesh, with
cleaning commonly accomplished using an ultrasonic liquid
bath.

Coolers. In force-feed lubrication systems,
the oil inlet temperature to the gearbox is controlled by
passing hot sump oil through a cooler. Such a cooler must be
capable of achieving the required oil temperature drop when
exposed to the maximum ambient air temperature anticipated for
the application. However, a generous temperature margin should
be applied during design to account for cooler
deterioration.

The two types of coolers used are liquid-to-liquid and
liquid-to-air. In oil-to-water (liquid-to-liquid) coolers, hot
oil gives up heat to cooler water, resulting in cooler oil and
hotter water. Where water is unavailable, radiators are used to
blow cooling air over oil tubes. However, air-to-oil
(air-to-liquid) coolers require larger envelopes than
oil-to-water coolers; in addition, hot days will limit the
amount of cooling a radiator can achieve.

Oil reservoir. The oil reservoir may be
integral with the gearbox or separately mounted and connected
to the gearbox by piping. The oil level in the reservoir will
vary from a maximum when the unit is shutdown and oil has
drained from lines and components to the minimum permitted
during operation.

At shutdown, when lines and components such as coolers and
filters drain back into the reservoir, the oil level will be
higher than the maximum operating level; thus, the reservoir
tank must have sufficient volume to accommodate the drain
backflow and still retain some air space at the top. To ensure
complete draining for cleaning and oil changes, the unit should
be fitted with a drain connection located at or near the bottom
of the sump. The oil pump suction line should be located
slightly above this reservoir bottom so that any sediment on
the bottom is not pulled into the pump suction line.

Oil return lines should be piped into the reservoir near the
maximum operating level away from the area around the pump
suction connection so that the incoming oil must travel the
maximum distance to the pump suction. By maximizing this dwell
time, the oil has more time to lose any entrained air before it
is again circulated through pumps, filters and coolers. To
facilitate reservoir inspection and cleaning, sufficiently
large access openings must be provided.

Breather. The gearbox breather is used to
vent pressure that may be built up in the gearing
unitsuch pressure may result from air entering the
lubrication system through seals or the natural heating and
cooling of the unit. When a cold gearbox starts up, the heat
generated during operation will cause air pressure to build
within the gearbox housing.

Piping. The lubrication system piping is
intended to distribute lubrication oil in accordance with
system design requirements and should be as simple and direct
as practical. The piping connections for a gearbox can cause
problems at assembly and startup since often the responsibility
for supplying piping and lubrication system components is split
between the gear manufacturer and user. If this is the case,
care must be taken to avoid piping complications at
installation.

It is good practice to have only one external oil feed
connection with all other oil passages placed inside the system
casingthis means that any slight leaks in the piping
connections will be internal and harmless. This also means
there will be less chance of damage during installation. In
all, the piping arrangement must be carefully designed to
minimize pressure drops and leak sources.

Lubrication monitoring. In providing
reliable service, the lubrication system must incorporate
sufficient sensors to allow continuous and complete system
condition monitoring.

Resistance temperature detectors (RTDs). RTDs allow
continuous temperature monitoring at key locations, such as oil
supply and drain temperatures, as well as sump temperature.
RTDs should be of the duplex type to obtain redundant readings
for accuracy or to supply a backup.

Temperature switches. Temperature switches
are typically used as a trigger to alarm or shut down the
gearbox prime mover when excessive temperatures are
experienced, and can be permissives for cold temperature
startup. These switches are typically located in the main oil
reservoir and lube-oil supply lines.

Pressure switches. Pressure switches are
used as a mechanism to trigger the operation of auxiliary
back-up pumps, as well as to initiate a signal for system
shutdown when pressure is lost at the main oil inlet to the
gearbox.

Flow switches. Flow switches are used as a
trigger to signal loss of flow below the minimum oil demand
requirements of the system.

Water detectors. Water detectors act as a
means to detect the presence of unwanted water in the
lubrication oil system.

Lubrication system maintenance. Given the
integral role that the lubrication system plays in overall
gearbox life and longevity, it must be continually maintained
so that the system is functioning at peak performance. It is
important to develop a systemic inspection method, condition
verification and documentation to avoid any unexpected
lubrication system failures, and ultimately equipment damage.
The following are areas of concern for maintaining a properly
functioning lubrication system.

Cleanliness. Dust, dirt, grit and wear
particles in the lubricant supply must be kept to a minimum.
Filters and strainers should be serviced regularly to avoid
circulating contaminants within the oil, as well as to avoid
excessive pressure drops that can reduce the quantity of oil
supplied to the gear drive.

Lubricant condition. The service life of a
lubricant is negatively affected by a number of factors,
including high temperatures, water and/or emulsions, solid
contaminants and operating environment. An oil sample should be
drawn from the oil sump at scheduled intervals and analyzed by
the lubricant supplier or a reputable maintenance provider. The
lubricant supplier should be consulted for typical oil
changeout limits for the particular oil used.

Sensor/switch settings. An annual check of
all switches and sensors should be performed to verify
operation as per lubrication system schematic specified
settings. System vibration and environmental conditions can alter
settings, ultimately affecting critical timing and initiation
of sensor functions.

Auxiliary pump function. Pumps and other
motorized accessories should be checked at scheduled intervals
to verify operability, proper oil delivery, pressures and motor
power draw. Relief valve settings should be checked to ensure
that the required oil delivery is supplied to the gear drive at
the proper pressure.

Flow and pressure check. Flows and pressure
drops at the cooler, filters and inlet to the rotating
equipment should be routinely monitored and recorded to
identify any adverse trends that might be developing.

Cooler condition. An annual check of cooler
condition is important to maintain cooler efficiency.
Water-cooled heat exchanger coolant ports should be checked for
any fouling or blockage. All sacrificial anodes should be
replaced. Air-oil cooler core fins should be checked and
cleaned of any dirt buildup that would affect heat transfer
efficiency.

Breathers. Oil breathers should be checked
frequently since they will become dirty. Any blockage in the
breather could potentially lead to leakage elsewhere in the
drive to relieve pressure.

Sound levels. The operating sound level of
the pumps should be routinely noted. Any increase in sound
level could indicate the presence of air in the lube system,
blockage at the pump intake, air leaks in the pump shaft seal,
worn or loose parts in the pump, filter blockage or high oil
viscosity from the pumped fluid being too cold.

Greased points. Some motors and pumps are
equipped with greased bearings that must be lubricated at
manufacturer recommended intervals.
HP

The
author

Jules DeBaecke is vice
president of engineering for Philadelphia Gear Corp. in
King of Prussia. He is responsible for maintaining and
enhancing Philadelphia Gear's leadership role within
the industry as world-class engineering experts. Mr.
DeBaecke joined Philadelphia Gear in 1981 as
engineering design manager for the Synchrotorque
(hydroviscous clutch) and Marine Divisions, as well as
manager of production engineering for all products.
Since then, he has also held positions as product
manager for the marketing and sales of marine,
synchrotorque, test stand, material handling and
high-speed product lines; and marketing manager. Prior
to 1981, Mr. DeBaecke was employed by the Naval Ship
Engineering Center, Philadelphia Division, where he was
responsible for research, development, test and
evaluation of US Naval main propulsion equipment, as
well as fleet machinery maintenance worldwide for
this equipment. At the project level, he was
recognized as the US Navy's clutch expert for equipment
installed on naval vessels including gas turbine-driven
cruisers and destroyers, nuclear submarines and
aircraft carriers. Mr. DeBaecke holds a degree in
mechanical engineering from Drexel University in
Philadelphia, Pennsylvania.

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